As known, LED sources, particularly if powerful, need to be used at an operating temperature below their limit temperature, in order to avoid a reduction in the flow of optical radiation emitted or even breaking of the sources themselves. For this purpose it is necessary for the junction temperature of the LED source to be kept below a predetermined limit value.
In order to keep the operating temperature of an LED source down, it is known to associate with the latter one or more passive heatsinks. In this case, the operating temperature of the LED source depends both on the current absorbed by it and on the amount of heat removed by the heatsink. For this last reason it is particularly tricky to control the temperature of an LED source of an external lighting unit of a vehicle, since the amount of heat removed by the heatsink depends on numerous external factors while the junction temperature of the LED source needs to be kept below a limit temperature irrespective of the external environmental conditions or other conditions that can in some way influence the efficiency of the heatsink. In a vehicle, the efficiency of the heatsink is for example significantly influenced by the environmental temperature, by the presence and by the features of an airflow that hits the heatsink (which depends greatly on the speed of the vehicle in the case in which the lighting unit is a front lighting unit), by the presence of external heat sources, like for example direct solar radiation.
In order to avoid overheating of the LED source irrespectively of the aforementioned external factors, it would be necessary to provide a large heatsink that can ensure correct and sufficient dissipation even when the air flow that hits it is practically zero, a condition that occurs for example in the case in which the vehicle is stopped. However, this would greatly influence the bulk and cost of the lighting unit. Some front headlights of vehicles of the state of the art are equipped with a dedicated cooling fan and a temperature sensor, so that it is possible to reduce the operating temperature when required. This solution, whilst reducing the dimensions of the heatsink, has the drawback of resulting in an increase in costs and power consumption.